(a) Field of the Invention
This invention relates to a method of dissipating the static charge that is built up by a dryer fabric operating in the dryer section of a paper making machine, thereby preventing adhesion of the paper to the fabric when it is transferred from one fabric to another. More particularly the invention relates to the addition of an anti-static agent to the coating material which is applied to the dryer fabric.
(b) Description of Prior Art
In a paper making machine, after the web of paper has been formed and partially dewatered in the forming and press sections, it is delivered to the dryer section where it passes, in serpentine fashion, over a number of heated rotating dryer cylinders and is held tightly against the cylinders by dryer fabrics. A conventional dryer fabric consists of an endless belt woven from either natural or synthetic yarns to form a relatively bulky fabric that will have good absorbent characteristics and high porosity. The yarns are woven closely together and sometimes in several plies to form a comparatively impermeable fabric. The fabric may have incorporated into it stuffer yarns of bulky mineral or organic fibres and is generally coated with a resin material to impart dimensional stability and resistance to degradation by hydrolysis and/or heat.
The function of a dryer fabric is to hold the paper web against the heated surfaces of the rotating dryer cylinders in order to promote more effective heat transfer to the web by partially eliminating a heat insulating layer of air which adheres to the surface of the cylinders. The fabric also serves to prevent the paper web from wrinkling.
Dryer fabrics run at speeds up to 3000 feet per minute and over metal rolls that are heated and dry with the result that, in spite of the moist environment due to water being driven off from the paper web, there is considerable build-up of static electricity in the system. This build-up is particularly evident in the dry end of the dryer section--that is in the last few sub-sections before the paper web is delivered to the stack of the machine.
There are two objectionable disadvantages caused by build-up of static electric charge on a dryer fabric. One is that when a machine is being started up and attempts are made to string the paper through the dryer by passing a tail of paper from one roll to another or from one subsection to another, as is common practice, the tail of paper tends to cling to the electrostatically charged fabric on or under which it is running and resists being lifted off for transfer. This results in loss of paper stock and time during start-up procedures. This disadvantage is particularly evident after a break in the paper or an extended temporary stoppage of the machine when the dryer cylinders remain hot and the fabrics have become dry. Another disadvantage is that personnel attending the machine can be subjected to startling and sometimes painful shocks.
Attempts have been made previously to dissipate static electric charge from dryer fabrics and these have included the addition of metallic strands in the yarns that are woven or the addition of finely divided carbon particles embedded in polyester yarn. Neither of these methods has proven successful. Attempts have also been made to drain away the static charge by various means of grounding components of the dryer equipment. This has not been entirely successful either because the static charge builds up between grounding elements and still is hazardous to personnel.